The referenced U.S. Pat. No. 3,949,401, HEGELER et al., describes an FM transmission system in which special recognition frequencies are used for special announcements which are not to be missed by the user of automobile radio receivers. Transmitters which radiate such special announcements can be recognized by radio receiver equipment by sensing an auxiliary carrier which is radiated in addition to the program modulation. A suitable frequency for the additional carrier, besides the program modulation, is 57 kHz which, in stereo transmitters, is radiated as the third harmonic of the 19 kHz stereo pilot tone, in synchronism therewith. The 57 kHz auxiliary carrier is phase-locked to the pilot tone of the 19 kHz carrier so that the zero or null crossings are synchronous, and in the same crossing direction. The auxiliary carrier is used additionally for the transmission of auxiliary information, hereinafter referred to as recognition, which are superimposed in the form of amplitude modulation on the auxiliary carrier. For a detailed discussion, the referenced U.S. Pat. No. 3,949,401, and the literature cited therein, is referred to.
One of the "recognitions is radiated together with the announcement. The respective recognition indicates that, during radiation over the FM transmitter, an announcement is being broadcast and, therefore, will be termed herein "announcement recognition", AR for short. An announcement recognition signal--an AR signal--corresponds to the signals described as the DK signals in the aforementioned U.S. Pat. No. 3,949,401. The AR signal is within a very narrow frequency band of 125 Hz, modulating the auxiliary carrier of 57 kHz with a predetermined percent of the amplitude of the auxiliary carrier.
The receiver which is arranged to operate with the system includes a 57 kHz detector and an amplitude demodulator and switching in the audio stage. The 57 kHz detector and the amplitude demodulator control the switching of the audio output. Various switching arrangements are possible, for example the amplitude of reproduction during the announcement could be raised to call specific attention thereto--for example to a traffic warning announcement; or, if the receiver is muted, a muting circuit is disabled; or, in a combined radio-cassette recorder, the audio section can be switched over from reproduction from the cassette to reproduction of the announcement when the announcement starts, and for switch-back to reproduction from the tape cassette when the announcement has terminated. Tape transport in the cassette can also be controlled to cause the cassette to stop and start in synchronism with interruption of its audio output.
The auxiliary 57 kHz carrier can provide further recognition signals; one further such recognition signal can be used to characterize the specific transmitting radio station, or a geographic region. All transmitters capable of radiating the announcements which are within a specific geographical region, for example, may be assigned the same region recognition, for short RR, and provide RR-signals, which correspond to the BK signals of the aforementioned U.S. Pat. No. 3,949,401. The traffic announcement within a region generally relate to the same geographical area. The region recognition signal modulates the amplitude of the auxiliary carrier continuously with a predetermined percent of the auxiliary carrier amplitude, for example 60%. The band width of the various region recognition signals, and their position with respect to each other is is so selected that, with the quality factor of more than 20, adjacent channel separation of more than 15 db is obtained. Within the available frequency band, 6 RR signal frequencies have been set in one system, and so relatively positioned that the harmonics of any RR signal fall outside of any other RR signal. Suitable frequencies for region recognition, that is, RR signals, are for example 23.75 Hz, 28.27 Hz, 34.93 Hz, 39.58 Hz, 46.67 Hz, 53.98 Hz, 63.61 Hz, 75.80, 98.96 Hz, and 122.85 Hz.
During an announcement, the auxiliary 57 kHz subcarrier is modulated by two recognition signals, namely the AR announcement recognition signal and the RR region or radio station recognition signal. When no announcement is being given, the auxiliary 57 kHz carrier is modulated only with the RR, the region recognition signal.
For further details of the system, reference is made to the aforementioned U.S. patent HEGELER et al. and to the applications Ser. Nos. 06/319,653, 06/319,654, 06/319,655, EILERS and BRAGAS, all filed Nov. 9, 1981.
The region recognition signal may also be used as a radio station recognition signal, based upon availability of frequencies so that, within any one geographical area, different transmitters may have different radio station recognition or RR frequencies assigned thereto.
The radio set may be constructed in various ways, for example to reproduce continuously radiated FM signals, or so arranged that the radio which is turned ON does not reproduce any audio signal, however, but only evaluates the AR or announcement recognition signal, radiated during an announcement, if the tuning section is tuned to a station which also radiates the RR or radio station recognition signal. This tuning may be manual or automatic. When no AR signal is being radiated, the radio audio reproduction may be low, or completely muted, or the audio section can be changed over to a differently tuned radio station, or to reproduce an audio program on a cassette. Cassette operation is interrupted during the presence of the AR signal, that is, during an announcement.
In a simpler set, the AR signal is evaluated only to provide an output indication, visible, or audible, or both, so that the operator of a vehicle is alerted to listen to the announcement. In dependence on the type of radio selected, tuning may be manual or automatic, for example by a "signal search" tuning section.
Radios of this type provide the opportunity to a vehicle operator to be continuously advised of traffic conditions.
Audio reproduction from the radios as known is over loudspeakers. If the vehicle carries passengers, it may be desirable for a passenger to continue to listen to a cassette program, or to a program being radiated by a station which does not carry the special announcements, while the operator of the vehicle may be interested in listening to the special announcement, which may relate to traffic conditions--although it could also contain other announcements, such as sport reports, for example.
Existing equipment does not permit individual selection of the programs by a passenger and a vehicle operator; rather, the passenger must listen to what the vehicle operator determines.